1. Field of the Invention
The present invention relates to the field of analog-to-digital conversion and, more particularly, to a technique for resetting state variables in a delta-sigma modulator of an analog-to-digital converter.
2. Background of the Related Art
The general technique of providing analog-to-digital (A/D) or digital-to-analog (D/A) conversion of signals is well known in the art. Generally, the sampling rate required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is known as the Nyquist rate. More recently, oversampling methods have been utilized for A/D and D/A conversion. In an oversampling type of converter, the sampling rate is much higher than the Nyquist rate. An oversampling technique is described in a reference titled "Oversampling Methods for A/D and D/A Conversion;" by James C. Candy and Gabor C. Temes; IEEE; pp. 1-25; 1992.
An advantage of using the oversampling technique is in the precision provided by the converter. With converters operating under the Nyquist rate for sampling, a certain amount of precision is required for the conversion. For example, in converting an analog signal into a 16-bit digital format, 16-bit precision is required. Accordingly, circuits will need to be designed having components which will meet this precision. In many instances, closely trimmed circuit components or precision matching (or compensating) circuits are required to meet the precision.
However, when sampling at a rate much higher than the required Nyquist sampling rate, the oversampling technique permits circuit precision to be relaxed significantly. For example, if the above A/D 16-bit oversampling converter implements an oversampling modulator, the modulator output can be a single bit output. The circuit precision needs only to meet this 1-bit output. Accordingly, closely trimmed circuit components are generally not needed. Additionally, 1-bit precision can be readily met by current generation CMOS (complementary-metal-oxide-semiconductor) components.
The disadvantage of using the oversampling technique is the added requirement that the output now needs to be reduced to the standard Nyquist rate at the eventual output of the converter. That is, the higher sampling rate now needs to be returned to the Nyquist rate. In the above 16-bit example, a multiple number of the 1-bit outputs will need to be combined to form a single 16-bit output, which output is equivalent to the 16-bit output from the Nyquist rate converter. However, the oversampling technique is preferred in many applications, since the cost savings in using less precise circuit components outweigh the additional digital signal processing needed at the back end of the converter.
One well known type of oversampling A/D conversion uses a modulator commonly referred to as a delta-sigma modulator. In a A/D converter (ADC) using a delta-sigma modulator, an integrator and a comparator are utilized at the front end of the converter to provide the quantization of the analog signal. Then, a low-pass filter and a decimator are utilized for digital signal processing to provide a corresponding digital signal at the Nyquist rate. However, the circuit precision of the analog circuitry can be relaxed, due to the use of the higher sampling rate.
When delta-sigma modulators are utilized, the modulator can be designed for higher than the first order of operation. Higher order operation of a delta-sigma modulator is desirable, since lower sampling rates can be utilized to obtain the same precision as operating the modulator at a lower order but with higher sampling rates. However, at higher order operation (notably, above the second order), stability is a significant concern. That is, the non-linear response of the delta-sigma comparator in the feedback path causes an unstable behavior. See "A Stereo 16-Bit Delta-Sigma A/D Converter for Digital Audio;" by D. R. Welland et al.; Journal of the Audio Engineering Society, vol. 37, pp. 476-486; June 1989; and "A Higher Order Topology for Interpolative Modulators for Oversampling A/D Converters;" by Kirk C.- H. Chao et al.; IEEE Trans. Circuits and Sys., vol. CAS-37, pp. 309-318; March 1990.
It is to be noted that the instability condition is different than an overload condition. In an overload condition, the modulator experiences a degraded signal-to-noise ratio when the input amplitude exceeds a certain value, but the modulator can recover when the overload condition is removed. Instability is also a function of the amplitude of the input signal, but in this instance (unlike the overload condition), the modulator cannot recover from an unstable behavior with the reduction of the input signal. Generally, instability occurs in third-order and higher systems. In order to return the system to its proper operating behavior, the state variables of the modulator need to be reset to values within a stable state space. Resetting the values to a zero condition will suffice.
In order to address the occurrence of an unstable condition in an order higher than the second order, a number of schemes have been devised. For example, in U.S. Pat. No. 5,012,244, an oscillation detect or is used to detect an occurrence of an oscillation condition in one of the integrator stages. The detection scheme is for the purpose of resetting the modulator once instability is sensed. Generally, a reset switch is placed across the input and output of the operational amplifier and the switch is closed to reset the integrator to a zero input condition. Resetting is necessary in third and higher order modulators, since recovery from an unstable condition is usually not possible with a reduction in the input signal amplitude. The present invention provides for an improved resetting scheme.